1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus which assists power required for steering operation by applying rotating force of the motor, more particularly, it relates to a power steering apparatus responsive to the vehicle speed.
2. Description of Related Art
There is such a conventional motor-driven power steering apparatus which drives a steering-assisting motor based on the result of detecting steering torque applied to the steering wheel and which assists the power required for steering of the vehicle by rotating force of the motor so that the driven can enjoy comfortable steering torque.
This conventional power steering is composed of a rack shaft which is installed by way of extending itself in the left and right directions of the vehicle body and which has both ends connected to the left and right wheels respectively through respective tie-rods and of a pinion which is engaged with the middle of the rack shaft and interlinked to the steering wheel.
There are those vehicles comprising rack/pinion steering mechanism performing steering operation by converting rotation of the pinion relative to the rotational operation of the steering wheel into the movement in the lengthwise direction of the rack shaft. These vehicles are classified into two categories according to the disposed position of the steering assisting motor. That is, in one vehicle the pinion shaft is further extended from the rack-shaft-engaged position and the steering assisting motor is installed so that the rotating force can be transmitted to the extended portion of the pinion shaft through an adequate reduction gear unit. In the other, an auxiliary pinion engaged with the rack shaft at a position in the direction of the shaft length apart from the above rack-shaft-engaged position and the steering assisting motor is installed so that the rotating force can be transmitted to the auxiliary pinion through an adequate reduction gear unit. Based on the number of pinions engaged with the rack shaft, conventionally, the former is called "single pinion" type and the latter "double pinion" type, respectively.
In operating the conventional power steering apparatus cited above, since reeling sense accompanies the steering sense at a high speed running, this conventional power steering apparatus controlled to diminish the steering assisting force in correspondence with the accelerated speed by making the steering assisting force respond to the vehicle speed so that the reeling sense can be suppressed.
Nevertheless, even in either of above-mentioned two types of the conventional power steering apparatus, since the rotating force of the steering assisting motor is transmitted to the extended portion of the pinion shaft or the auxiliary pinion through the reduction gear unit, when the driver operates the steering wheel and then returns it at a high speed running, the steering wheel may excessively turn itself due to inertia force of the steering assisting motor, thus generating sense of awkwardness of steering operation.